Study On The Carbon Dioxide Corrosion Of Tube And Casing In Downhole

The　basic　theory　of　carbon　dioxide　corrosion　is　ascertained　in　oil-gas　field.　It　will　conduce　to　guide　OCTG-materials　optimizing　choice　and　development　of　new　material,　draft　out　sensible　anticorrosion　project　during　petroleum　and　natural　gas　production.　But　so　far　fundamental　research　of　carbon　dioxide　corrosion　is　a　bottleneck　in　oil-gas　field　corrosion　scope.　The　structure　characteristics　of　corrosion　product　film　of　the　common　tube　and　casing　materials　(N80　1Cr　3Cr　and　13Cr)　,　local　corrosion　mechanism　and　the　shear　stress　of　materials　in　test　equipment　are　studied　,and　the　CO2　general　corrosion　rate　prediction　model　of　carbon　steel　is　set　up　through　corrosion　simulating　test　in　high　temperature　and　high　pressure　environment　and　theoretical　analysis.　The　test　results　indicate　that　the　degree　of　corrosion　of　the　common　tube　and　casing　materials　is　3Cr>1Cr>N80>13Cr　in　our　proposed　corrosion　environment.13Cr　steel　displays　very　fine　anticorrosion　ability,　but　low-chrome　steels　are　corroded　more　severely　than　N80　steel.　Furthermore　the　corrosion　rates　of　materials　in　crude　oil　are　small,which　verifies　that　there　are　some　inhibiting　corrosion　ingredients　in　the　crude　oil.　The　D/max-rA　diffractometer　and　S　-　450　scanning　electron　microscope　X　ray　energy　chromatic　aberration　spectrometer　EDAX9100/60　are　used　to　study　the　structure　characteristic　of　corrosion　products　film.　With　the　result　that,13Cr　steel　displays　very　strong　CO2　anticorrosion　ability　and　the　corrosion　products　are　less　on　it,　because　of　the　high　electrode　potential　of　13Cr　steel　and　not　the　formation　of　protective　corrosion　products　film.　For　the　low-chrome　steels,　with　Cr　content　increasing,　the　crystal　size　increases,　the　stability　of　corrosion　product　film　becomes　badly,　and　the　corrosion　product　film　is　destroyed　more　easily.　Moreover,　the　destruction　of　corrosion　products　film　of　the　low-chrome　steels　is　more　severe　than　carbon　steel's.　The　local　corrosion　mechanism　is　studied　from　pitting.　For　the　N80　steel,　there　are　many　small　pits　on　it,　and　their　depths　are　shallow　too,　the　flaws　of　corrosion　product　film　result　in　the　pitting　of　N80　steel.　The　pitting　of　1Cr　steel　is　more　deep　and　results　from　the　destruction　of　corrosion　product　film　by　fluid　shear　stress.　The　pit　depth　of　3Cr　steel　is　very　deep,　its　corrosion　product　film　severe　destruction　makes　material　surface　more　rough,　further　leads　to　severe　pitting.　With　the　Cr　content　increasing,　chromium　compounds　will　increase　in　corrosion　product　film,　which　make　its　inner　stress　increase　and　intensity　decrease.　And　gas　pressure　makes　inner　stress　enhance.　The　lid　of　corrosion　film　is　torn　away　more　easily　by　the　mechanical　forces　of　the　turbulent　flow.　The　attacking　ions　are　adsorbed　more　easily　in　the　position　free　from　corrosion　film,　which　make　ionic　concentrations　uneven　on　the　film　and　further　make　concentration　difference　cell　form.　The　concentration　difference　cell　can　make　materials　corrosion　quickly.　After　the　corrosion　film　torn　off,　the　galvanic　cell　between　the　film-free　bottom　and　the　film-covered　metal　results　in　severe　local　corrosion.　For　the　study　on　shear　stress　which　material　surface　suffers　in　test,　FLUEND　software　is　used　to　simulate　shear　stress.　Under　the　three　corrosion　conditions,　shear　stress　is　4.738　Pa,5.236　Pa,5.209　Pa　respectively.　At　the　same　time,　it　is　found　that　the　shear　stress　not　same　in　the　different　position　because　of　eddy　from　turbulent　flow　and　edge　effect.　The　results　are　verified　by　comparing　test　results　with　general　corrosion　rate　prediction　model　results　which　use　FLUENT　calculating　results.　On　the　basis　of　both　Norsok　Standard　(CO2　corrosion　rate　calculation　model)　and　De　Waard's　scale　factor,　in　this　text,　the　CO2　general　corrosion　rate　prediction　model　of　carbon　steel　is　set　up　as　follows.　( )( )CO220.146 0.0324log0.62 wCR K CO 19pHtft = t ×f ×??? τ??? +×f scale×f　where　Kt　　is　constant　for　the　temperature;　f CO2　is　the　fugacity　of　CO2　(bar);　τw　is　wall　shear　stress(Pa);　f scale　is　scale　factor　at　temperature　t;　f ( pH )t　is　the　pH　factor　at　temperature　t.　The　model　verified　under　100℃　and　110℃,　and　the　errors　with　test　results　are　0.85%　and　7.11%respectively.　Comparing　with　other　calculation　model,　this　model　has　no　any　limitation　to　corrosion　system　construction　because　FLUENT　software　is　recommended　to　compute　wall　shear　stress,　and　pH　can　receive　easily　and　correctly　from　chart.　At　the　same　time,　temperature,　CO2　pressure,　wall　shear　stress,　pH　and　corrosion　products　film　are　considered　in　this　model.